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Can starter battery & house battery share negative on car’s metal chassis?

James Chung

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Jun 28, 2021
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Hi, can we actually use a vehicle’s metal chassis on both starter battery(main) and house battery(auxiliary)

Plan to use alternator as an option to charge house battery. Will either use DC-DC charger or VBS (voltage sensitivity relay) to isolate starter battery and house battery.

Reason to isolate is because starter battery is AGM battery and house battery is LiFePO4. And of cause other reasons such as avoiding house battery draining starter battery.

Definitely not going to connect both starter battery and house battery’s B+ together. But hope to connect both battery’s B- on the chassis for easier wiring on accessories.

Your kind assistance is much appreciated,
James
 
I don't see any reason why you couldn't do it .... but personally, I wouldn't just because of the increase of chassis rust due to galvanic corrosion.

Chassis rust is usually the reason I have to give up on a vehicle. If you live in an area where ice melt isn't used on the roads it may not be as much of a concern.
 
I wouldn’t use a vehicle ‘chassis’ as a ground point.

The battery cables to the block are probably heavy enough, but most -probably all- vehicles do not have a high amp negative(-) cable to the frame if there is a frame at all, AND the battery-to-body is designed to carry only the vehicle electrical needs at best. Most components are provided with hard-wired neg(-) connections and do not rely on the body sheet metal for a path to neg(-) except perhaps a single point near a PDC or fuse box.

Never minding that an LFP battery may not like the vehicle alternator’s charging characteristics anyway, the negative connection to the aux battery should be of sufficient capacity for the alternator’s amp potential. If your alternator is 100A, you should use pos(+) and neg(-) cables that can support 100A. If you’re using a DC2DC charger that is say 30A, you should use 30A cables.

I’d use the DC2DC charger with a lifepo battery. Smaller wire, potentially longer battery life

My opinion anyway
 
I don't see any reason why you couldn't do it .... but personally, I wouldn't just because of the increase of chassis rust due to galvanic corrosion.

Chassis rust is usually the reason I have to give up on a vehicle. If you live in an area where ice melt isn't used on the roads it may not be as much of a concern.
Why would a second battery electrical system cause increased galvanic corrosion?

Seems like to me, having multiple grounds will just cause problems. As long as the power side of the electronics are isolated from each other, that's the real issue. But each system will need some kind of ground- so both the LiFePO4 system will need a ground as will the lead system. Keep them the same and very solid. Floating grounds have really weird problems.
 
I wouldn’t use a vehicle ‘chassis’ as a ground point.

The battery cables to the block are probably heavy enough, but most -probably all- vehicles do not have a high amp negative(-) cable to the frame if there is a frame at all, AND the battery-to-body is designed to carry only the vehicle electrical needs at best. Most components are provided with hard-wired neg(-) connections and do not rely on the body sheet metal for a path to neg(-) except perhaps a single point near a PDC or fuse box.

Never minding that an LFP battery may not like the vehicle alternator’s charging characteristics anyway, the negative connection to the aux battery should be of sufficient capacity for the alternator’s amp potential. If your alternator is 100A, you should use pos(+) and neg(-) cables that can support 100A. If you’re using a DC2DC charger that is say 30A, you should use 30A cables.

I’d use the DC2DC charger with a lifepo battery. Smaller wire, potentially longer battery life

If you are not using a DC2DC I would absolutely use the vehicle body as ground, exactly because of the reason you are describing - it's a poor conductor.

LifePO4 batteries have incredible low internal resistance - something alternator doesn't know how to deal with and potential overheat.
That problem is easy cured with adding a little bit of resistance to the circuit. Hence including the vehicle body.

Do not ever connect a Lithium battery alone to a alternator. Always parallel a Lead battery and go through the vehicle frame.

The vehicle body is not floating ground in a charging system - that's only interesting for communication (data) and audio circuits.
Galvanic corrosion a thing of the past and that is not happening anymore. (at the terminals, yes but not on the frame)

Plan to use alternator as an option to charge house battery. Will either use DC-DC charger or VBS (voltage sensitivity relay) to isolate starter battery and house battery.

I'm using the Relay option. Measure !! that your alternator is not putting out more voltage then your chosen battery BMS can take in. Some trucks put out 15.5V and some are capped at 14.4V (ideal for lithium with a charge cutoff of like 14.8V )

Get a clamp on amp meter when put the system together - start with the lithium charged at like 80% before you actuate the relay the first time. - Measure the amps.

Slowly drain the battery and measure the amps flowing at various states of charge. (and engine revs)
Compare those values with your alternator rating.

If you get within 80% of the maximum of the alternator for prolonged periods (15-20min) - get a larger alternator. Still cheaper then a large DC2DC charger
 
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A DC to DC battery charger like the Victron Orion is the safest way to charge lithium because it can be programmed for the correct charge voltages and will terminate charging correctly. In addition the loading on the vehicle systen is known and within the safe working envelope of the regulator for reliability.
The vehicle uses the chassis/body for negative return for the alternator, starter and most other equipment on the vehicle. if you connect direct to the alternator or use a DC to DC charger (non isolated) you will be using the chassis/body for negative return back to the alternator.
Experience has shown no ill effects using a correctly made connection to the body/chassis, there is often a suitable point used by the negative strap from the engine battery. If the vehicle has 'smart alternator' technology there will be a current sensor in the battery negative path. Most manufactures recommend any additional current draw from the system uses chassis/body point for negative rather than the negative battery post.

The vehicle will have a heavy duty cable/braid between engine block and chassis/body rated for the hundreds of amps required by the starter motor, so provided its in good condition should not cause issues.

Note that a 30 amp DC to DC battery charger rated at 30A will pull more current from the engine battery/alternator to make up for conversion losses and to 'step up' the voltage if needed. The cable should be rated for this, say 40 amps and fuse/circuit breaker, 50 amps at the end of the cable to each battery.

Mike
 
If you are not using a DC2DC I would absolutely use the vehicle body as ground, exactly because of the reason you are describing - it's a poor conductor.

LifePO4 batteries have incredible low internal resistance - something alternator doesn't know how to deal with and potential overheat.
That problem is easy cured with adding a little bit of resistance to the circuit. Hence including the vehicle body.

Do not ever connect a Lithium battery alone to a alternator. Always parallel a Lead battery and go through the vehicle frame.

The vehicle body is not floating ground in a charging system - that's only interesting for communication (data) and audio circuits.
Galvanic corrosion a thing of the past and that is not happening anymore. (at the terminals, yes but not on the frame)



I'm using the Relay option. Measure !! that your alternator is not putting out more voltage then your chosen battery BMS can take in. Some trucks put out 15.5V and some are capped at 14.4V (ideal for lithium with a charge cutoff of like 14.8V )

Get a clamp on amp meter when put the system together - start with the lithium charged at like 80% before you actuate the relay the first time. - Measure the amps.

Slowly drain the battery and measure the amps flowing at various states of charge. (and engine revs)
Compare those values with your alternator rating.

If you get within 80% of the maximum of the alternator for prolonged periods (15-20min) - get a larger alternator. Still cheaper then a large DC2DC charger

You missed the picture as far as the chassis is not likely to be connected to the alternator and battery with a large enough cable.

Sure a lot of people do it. I’ve fixed a number of examples of wiring issues and often their resultant fires. It’s just too easy to do well instead of taking the chassis shortcut/guess/risk imho.

someone mentioned floating ground- electrical engineering uses this principle sometimes for several intended/intentional purposes. There’s another kind of ‘floating ground’ and that’s the unintended/unexpected path electrons may take when something else (the intended path) ain’t working right.

With a proper neg cable to the block or batt instead of a body ground THAT is the path of highest potential.
 
Every RV uses the chassis to connect the negative terminal for both the house and chassis batteries. The positives are connected through an isolator so the alternator will charge both. Many MH have a boost start switch that will connect the house and chassis positive should the chassis battery get low and fail to crank the main engine.
 
^^^ that’s motor home RVs.

Most trailer campers (all?) I’ve worked on have a frame-connected ground wire somewhere but the batteries to converter/inverter are direct with both pos and neg cables.
I’ve seen ‘dump trailers’ smoke wiring in the tow vehicle or burn the insulation off the neg trailer-vehicle connection. Trucks with winches have under hood fires.

I’m not depending on any frame ‘ground’ above 8A. Ymmv
 
For a better ground path on trailers, maybe people should be using conductive grease on the mechanical connections. That's a pretty hefty potential path.
 
You missed the picture as far as the chassis is not likely to be connected to the alternator and battery with a large enough cable.

I never saw a vehicle with not a sufficient enough ground strap to the chassis. (aside of deterioration) Usually all the starter amps pass through the ground strap - and that's usually a lot more then 100A.

Motor driven RVs are usually built on a Massive Steel C Channel frame. Which is used as the reference "ground" - you are talking about adding a second negative from the engine directly to the battery. That alternative bath you are proposing is exactly what you describe you want to avoid a sentence earlier ?
Just get a bigger ground strap if you don't trust the one you got underneath there.


But I agree on the positive side, sometimes the cable coming from the alternator to the battery - are sometimes undersized. That's a potential fire hazard.

For my relay charging system I added a 0/2 wire directly on the 180A alternator. - It feeds into a 150A Fuse- 200A relay - another 150A Fuse and then going to the house battery bank.

That's definitely worth improving on the charge cable.
 
Please don't use a frame ground. Not only is it inefficient, but it introduces additional variables and points of failure into the system.

Either your alternator can handle a direct attach, or it can't.

If it can't, use a DC-DC converter with current limiting between the alternator and the external batteries. You could repurpose an MPPT for that purpose if you have one lying around unused, or want to upgrade one you're already using for solar.

About the only thing in modern cars that still uses frame ground is the starter, and that's only because they don't want to pay for the copper to connect it directly, and it's used so little compared to the rest of the systems that the inefficiency isn't a burden. Further, the path between the battery and the starter is very well defined, and often doesn't go through the frame, but through the engine block, with a ground wire directly from the battery to the engine.

Almost every other component has a negative wire, and does not use frame ground. It's still supported, and you can use it, but the latest anti-corrosion treatments mean that bonding wires have to be used, and even those present a higher resistance than is useful.

If you do decide to go with a frame ground, make sure you explicitly understand the current path and every fastener, part, and ground wire between the battery/alternator, and the frame ground point, and check them for corrosion every few hundred hours of use.
 
About the only thing in modern cars that still uses frame ground is the starter, and that's only because they don't want to pay for the copper to connect it directly, and it's used so little compared to the rest of the systems that the inefficiency isn't a burden. Further, the path between the battery and the starter is very well defined, and often doesn't go through the frame, but through the engine block, with a ground wire directly from the battery to the engine.
Cars are a different story.

Motor driven RVs (Class A and C and older B) are Foam boxes built on a Metal Platform chassis. Everything there is connected down the the C-Channel frame.

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The batteries in RVs are placed in various random places - I've seen everything from in behind the front bumper to under the stairs, till all the way in the back underneath the bed.

The all have a heavy cable going to starter - and the second cable going to the frame.
 
I never saw a vehicle with not a sufficient enough ground strap to the chassis.
Really?
That alternative bath you are proposing is exactly what you describe you want to avoid a sentence earlier ?
An intentional favored path isn’t a random floating ground potential. It’s a fixed neg(-)
That's definitely worth improving on the charge cable.

try being in the trailer/camper/truck/towing install industry. The straps you find ‘sufficient’ aren’t, and when unrelated things actually smoke when certain accessories or solar are installed until you install a huge secondary or direct ground that’s more than enough indicator to me that automotive grounding is generally insufficient. It’s sufficient for oem; we aren’t talking oem

I did state motorhomes are different and intentionally so.

Heavy trucks, Class C RVs etc are a different story. As shown above
 
Why would a second battery electrical system cause increased galvanic corrosion?

Seems like to me, having multiple grounds will just cause problems. As long as the power side of the electronics are isolated from each other, that's the real issue. But each system will need some kind of ground- so both the LiFePO4 system will need a ground as will the lead system. Keep them the same and very solid. Floating grounds have really weird problems.
Maybe mentioning that this bond to ground is not a load circuit, it’s a safety ground, is a good idea. There shouldn’t be current flowing through it on a regular basis. In the event the chassis system (or house, hopefully not though) uses the chassis as a return circuit, is an exception.

That said, i have a ground bar for the house circuit, with a single 4/0 ground cable bonded to the chassis on the same post as the chassis battery negative. I chose a common point because I will be adding a dc dc charger, and I don’t want to have to worry about isolated vs not, especially if i end up with something of questionable quality or reliability on the dc dc charger side.

Furthermore, all my house circuit components are isolated from the chassis at their mount point; and, where required, have their chassis connected to the ground bar previously mentioned. All house components have a negative wire for the return path, and therefore do not use the chassis as a return path for any part of the circuit.

If there was no DC DC charger or you spent money on a quality UL certified isolated dc dc charger, a common ground wouldnt be necessary (assuming no other complexities of the system required it).
 
Wait- are people really saying that a steel frame is not capable of pushing multiple hundreds of amps of current?

BTW, the real source of multiple hundreds of amps of current is the battery, not the alternator. And the alternator is grounded to the engine, which is grounded to the frame. And the battery is grounded to the frame.
 
Really?

An intentional favored path isn’t a random floating ground potential. It’s a fixed neg(-)


try being in the trailer/camper/truck/towing install industry. The straps you find ‘sufficient’ aren’t, and when unrelated things actually smoke when certain accessories or solar are installed until you install a huge secondary or direct ground that’s more than enough indicator to me that automotive grounding is generally insufficient. It’s sufficient for oem; we aren’t talking oem

I did state motorhomes are different and intentionally so.

Heavy trucks, Class C RVs etc are a different story. As shown above
Most heavy duty trucks run a ground wire from alternator to central ground point, same with the starter. And just about every circuit ground is ran to the central ground in some way, either thru a bussbar with a large cable to the central ground location or a separate wire.

Part of the reason is the aluminum cab but the other is to ensure an excellent ground over the lifetime of the truck along with prevention of arcing ground circuits (stray ground). This is important for trucks that haul hazardous material.

The best grounding is one that protects the circuit from resistance which a frame can not do as things age. The grounding should also be tied to the frame and motor to protect the circuit in case a short occurs. This is accomplished thru a central grounding stud attached to the frame.

For the OP, I would run a ground separate from the frame but tie in the frame in case of a short.
 
This whole topic is like a religion debate. I recently went through this with a DC/DC charger installation on my sprinter. There is a outfitters guide published by Mercedes that talks about proper ground points and there are at least a couple located on the chassis. The sprinter is not technically a body on frame vehicle, but it has a beefy frame structure as part of it's unibody.

I looked at one of the "approved" locations for grounding near the rear and it was a small stud with small wires connected to it. I was installing 2 Orion chargers that can pull ~80a, so I elected to create my own ground point near the "official" one. I used the following


This is a hotly contested issue in the custom car stereo world where they can be pulling huge amps and ground loops can cause big issues. I probably wasted 2+ hours of my life reading the debates of running a negative to the battery(or other location) vs. chassis ground. In the end, it was my determination that the chassis ground is a better approach as long as you make the effort to do it right. Time will tell.

on many modern vehicles, you don't want to go direct to the battery negative because there are often current measuring devices that you may effect if you do that. So, you are basically relegated to some form of chassis ground. It's really a debate on location. I was running a positive 2AWG wire to the positive bus bar under my drivers seat, but there isn't really a negative bus bar equivalent (just the approved chassis ground points). Running a wire to the front didn't provide any better options than the approved points at the back. It's not as if 20+ feet of 2AWG is going to do any better job of carrying the current than the frame.

The one point of agreement I see - everything should go back through the same ground path as much as possible. Every accessory you have should have it's own positive and negative run feeding back to the same location whenever possible. For my "house" 12V system, I have a lynx distributer that has a positive and negative bus bar and that is tied to chassis ground at the same point my orions are. So, the house and chassis 12v systems ultimately share a common ground. Since my orions are the isolated versions, I didn't have to put my house system onto the chassis ground because of them, but I had to do it in order to measure starter battery voltage and also to charge my chassis battery via a trickle feature on my inverter. So far, everything seems to be working fine and there
 
It seems some folks dont quite understand unibody vs on frame vehicles.
any on frame vehicle (full size rv, Jeep wrangler, pickup truck is built on a frame with 2 metal beams, those are the best ground doesnt matter where you attach as long as its solid. And yes the OP is right most fires are caused not from initial install but from a ground or power wire coming loose to an amplifer, winch etc that was ran to the battery instead of properly to the vehicle frame.
 
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